Taste helps to distinguish compounds that an animal needs from those that might be toxic. The psychological property that the brain attaches to taste afferent activity that accomplishes this distinction is called hedonic tone and, absent learned effects, its behavioral manifestation is ingestion or rejection. A positive hedonic tone, i.e. palatable taste, stimulates feeding and is one of many dietary factors associated with excess energy intake and, in the long term, may lead to obesity, diabetes, and hypertension. The object of this project is to understand how the neural systems of ingestive behavior control taste processing in the first and second central synapses of the ascending gustatory system. The behavioral response to taste information can be modulated by experience and immediate physiological state. For example, a learned taste aversion switches the response from ingestion to rejection. This fundamental behavioral change characterizes hunger and satiety. The sensory information from the oral cavity that guides ingestive behavior and the motor neurons that produce it are located in the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN). Integration of afferent visceral and taste information likely occurs in the brainstem independent of forebrain structures, because decerebrate rats, which lack connections between the forebrain and brainstem, can switch their response from ingestion to rejection following treatments that induce satiety. Nevertheless, learned control of ingestive behavior and some forms of unlearned control (e.g. nutrient deficiencies) requires connections to the forebrain. Coincident with these behavioral changes in food acceptance is selective alterations in the responses of NST and PBN taste cells. Using electrophysiological, neuroanatomical, neurochemical, and lesion analysis, this project will test hypotheses about the centrifugal control of taste processing in the brainstem. Specific experiments examine (1) differences between the influence of forebrain activity on NST and PBN taste neurons, (2) the route through which descending forebrain projections modulate PBN taste neurons, and (3) the neurochemicals released in the PBN by activation of descending forebrain inputs. Specifically, the neurophysiological mechanism underlying changes in ingestion and rejection behaviors might involve central modulation of afferent neural activity that conveys the hedonic character of a taste stimulus.